Modern transceiver integrated circuits may contain on-chip coils such as inductors and Balun transformers. These coils can produce very efficient and linear systems especially on the transmitter side.
In order to up-convert or down-convert baseband signals to or from Radio Frequency (RF) signals, many transceiver circuits also have a local oscillator running at a RF oscillation frequency on chip. A Voltage Controlled Oscillator (VCO) can be implemented as an on-chip local oscillator which uses an inductor in parallel with a variable capacitor coming from a varactor for tuning the oscillation frequency.
The transceiver circuits work with power amplifiers and act to filter out power amplifier output signals at unwanted frequencies before transmitting the output signals. Power amplifiers in certain classes such as D, E, F, etc. are digital power amplifiers and use digital signals to create sinusoidal RF waveforms. These digital signals contain square waves which are rich in harmonic frequency components. The harmonic signals can interfere in the far field with passive on-chip components, such as the VCO. The harmonic signals may cause magnetic and/or electric coupling to coils in these components, which may lead to unfixable system disturbance in some applications. An example is the potential coupling between the digital power amplifier coil(s) and a VCO inductor, including the situation when transmitter harmonic frequencies substantially coincide with a VCO oscillation frequency or harmonic frequency, the transmitter harmonic signals may cause serious VCO pulling, i.e. frequency variations in the oscillation frequency, and poor spectral purity.
There is therefore a need for an improved circuit and system that help reduce magnetic and/or electric coupling in transceiver circuits.